The main components of a drive train of a motor vehicle are a drive engine and a transmission. The transmission converts the tractive force provided by the drive engine. The components of the drive unit may be controlled via an oil volume flow provided by a hydraulic pump of the drive train. The oil volume flow, provided by the hydraulic pump, is further used for cooling the components of the drive engine.
For cost reasons, a fixed displacement pump is typically used in the drive engine of a motor vehicle for providing the oil volume flow, which is used to control and cool the components of the drive engine. In these cases, the input speed of the fixed displacement pump is adjusted, depending on the speed of the drive engine of the drive train, so that the oil volume flow, provided by the hydraulic pump, is dependent upon the speed of the drive engine. In order to avoid inadmissible heating in transmissions, for example in automatic transmissions, which comprise a hydrodynamic torque converter, a hydraulic pump has to provide a high oil volume flow even when the drive engine is running at low speed. Consequently, when the drive engine is operating at high speed, the hydraulic oil volume flow, provided by the hydraulic pump, is too big. The excess oil volume flow is released by pressure limiting valves, whereby hydraulic energy is converted into heat and consequently lost.
A drive for a fixed displacement pump of the drive train of a motor vehicle is known from DE 38 12 412 A1. The drive comprises a planetary gear, namely a planetary gear with an input path and two output paths, the output paths acting upon one another with respect to their speeds. Different assemblies may be driven via both output paths.
With this in mind, it is the object of the present invention to develop an innovative method for operating the hydraulic pump of the drive train of a motor vehicle having a transmission and a drive engine, as well as an innovative drive train.